1. Field of the Invention
The present invention relates to a method of manufacturing various structural parts of machines required to be especially high in surface hardness. This method is used, for example, in making various automobile parts, especially manufacturing case-hardened parts with little distortion in heat treatment and superior strength in bending fatigue by specifying the chemical composition of the steel material used and the conditions of heat treatment. The invention also relates to a method of manufacturing case-hardened parts whose dimensional precision is high due to minimun distortion in heat treatment such as automobile parts, e.g., automobile gears and the like. The method of the present invention is, however, not limited to manufacturing methods of automobile parts and is to be applicable to those all structural parts for the machinery whose fatigue strength must be excellent.
2. Description of the Related Technology
As automobiles have now come to be widely used in all corners of the globe, their influence on environmental problem is now being reviewed from a global point of view and today improvement of the output as well as fuel cost of automobile engine is being taken up urgently. As a means of solving this problem, it is now considered necessary to put emphasis on enhancing the strength and weight-reduction of automobile parts. To date, the present inventors have mede various attempts to improve the mechanical strength of parts, especially automobile gears and shafts. As such gears and shafts are cited, for example, transmission gears (final gears, for instance), differential gears (side gears, pinion gears etc.), drive gears of camshafts, hub sleeves and shafts. As the prior arts made from the above point of view may be cited, among others, the technique for preventing intergranular oxidation in the carburized layer by lowering the silicon content of steel (Japanese Patent kohkoku 32777/1980), the technique for preventing incomplete hardening of the surface of carburized layer by increasing the Mo content of steel (Japanese Patent kohkai 306545/1989) and the technique imparting the residual stress to the gear's root of tooth by shot peening after carburizing and tempering and improved steels of such kinds, represented by SCM420, SNCM420, etc are already marked.
Car users have expressd a strong desire for reducing of gear noise through running, this desire being stronger as car performance in general has been steadily improved.
The present inventors, therefore, made careful studies to trace the cause of gear noise and found out that the gear noise can be reduced substantially depending on the accuracy of size of the gears. An object of the present invention is, therefore, to provide a structural steel parts which are required to have a quite high hardness and are small in the distortion caused by the heat treatment. Another object of the present invention is to provide a method of manufacturing a case-hardened part excelled in dimensional precision as well as in bending fatigue strength due to reducing heat treatment-caused distortion such as a gear. Still a further object of the present invention is to provide an invention that can reduce the heat treatment-caused distortion of gears so as to provide both high precision and high strength through a study of steel's chemical composition and its metallographic structure. It is essential to use gears made of case-hardened steel of a specified chemical composition and do heat treatment under specific conditions.